The film was not exposed properly. Either the film was loaded with emulsion on the back. Or the film never went through the camera.

I had one incidence like this. I bought a MF camera, with a film inside. So I removed the film back and rewound it in the change bag. After several month, I took out the film and loaded into my camera. I spent a couple of hours on the trail, but the 6x4.5 camera would not load at frame #12. So I had to change to another film back with a new roll of film.

After I got home and opened the film back in change bag, I found that the film was already rewound to the end. So I loaded the supposed exposed film again. Since there is no tape at the end (front in my case), the film was never exposed. It just stayed with the spool and would not win when the spool area is full.

It did not harm me much since I was only testing that camera. But I did waste some of my time.